The "magnetoresistance" (MR) of a material is the resistivity .rho.(H) of the material in an applied magnetic field (H) minus the resistivity .rho..sub.0 of the material in the absence of the applied field. This difference .DELTA..rho. is typically normalized (e.g., by dividing by .rho.(H)) and expressed as a magnetoresistance ratio in percent.
Recently, giant MR ratios were observed in mixed metal oxides, exemplarily in La--Ca--Mn--oxide and in thin films of La--Ba--Mn--oxide.
Co-assigned U.S. patent application Ser. No. 08/228,168, filed Apr. 15, 1994 by S. Jin et al., (which is a continuation-in-part of Serial No. 08/154,766, now abandoned) which is incorporated herein by reference, discloses mixed metal oxides that can exhibit a negative MR ratio of 200% or more (in absolute value) at room temperature, typically attained at relatively high fields, e.g., at 6 Tesla. The '766 application also disclosed a method of making such material. Co-assigned U.S. patent application Ser. No. 08/187,668, filed Jan. 26, 1994 by S. Jin et al. and also incorporated herein by reference, discloses improved MR materials based on La--Ca--Sr--Mn--oxide or La--Ca--Ba--Mn--oxide, which can exhibit more sensitive MR behavior in low magnetic fields. See also S. Jin et al., Science, Vol. 264, p. 413 (1994), which shows very high MR ratios in excess of -100,000% at.about.77K. The paper inter alia also discloses that very high MR values of La--Ca--Mn--oxide typically are observed only in. epitaxial films on single crystal substrates.
For many technological applications of the mixed oxide materials it would be advantageous if high MR could be obtained from polycrystalline or near-polycrystalline samples of the materials. This could result in reduced cost. Furthermore, it could simplify design, since many otherwise desirable single crystal substrates do not have a lattice parameter that can support epitaxial growth of the relevant mixed oxides. This application discloses such mixed oxide material.